1. Field of the Invention
The present invention relates to a method for utilizing three waste streams to sequester flue gas constituent, and more particularly the present invention relates to a method for sequestering carbon dioxide and sulfur dioxide contained in flue gas by contacting the emissions with waste water brine pretreated with bauxite residue.
2. Background of the Invention
Many researchers suspect a link between global climate change and atmospheric concentrations of carbon dioxide (CO2). Studies have shown a correlation between a rise in atmospheric CO2 and increasing mean global temperature since the advent of the industrial era. In order to decrease the impact of CO2 on global climate, several strategies are under development that will potentially remove CO2 from the atmosphere or decrease CO2 emissions.
Principle modes for carbon management include: (i) increasing the efficiency of energy conversion; (ii) using low-carbon or carbon-free energy sources; and (iii) capturing and sequestering anthropogenic CO2 emissions. The latter strategy termed “CO2 sequestration” permits continued use of fossil fuels for the generation of electric power while ensuring CO2 emission reductions, and has gained increased attention in recent years. Various CO2 sequestration options include oceanic-, terrestrial-, and geologic-containment, and the use of advanced biological- and chemical-approaches. Each of these options has significant technical and economic hurdles that need to be addressed before being considered for full-scale application.
Geologic sequestration involves the capture of CO2 from large point sources (such as fossil fuel-fired power plants) and the long-term storage of CO2 in underground, brine-bearing geologic formations. Brine is an attractive reactant in CO2 sequestration schemes inasmuch as 20-30 billion barrels of saline wastewater are produced annually in the United States as a byproduct of enhanced oil and gas recovery. About 65% of this water is reinjected into reservoirs for pressure maintenance, and the remaining water is treated and discharged into surface water. In Pennsylvania, for example, a typical treatment cost for this wastewater can be as high as $3.00/barrel. However, the pH of subsurface aquifer brines is typically low (approximately 3-5). The primary issue affecting solubility trapping is the limited absorptive capacity of brine. This low pH precludes dissolution of CO2 and prevents carbonate formation.
Over 70 million tons of bauxite residue are generated annually when aluminum is extracted from the principal ore called bauxite (Aluminum Association, 2000). The residue is primarily comprised of iron and titanium oxides, silica, calcium carbonate, and unrecoverable alumina and caustic soda (NaOH), and, as such, is highly alkaline. The pH of the liquid reaches values as high as 13.5 (the hazardous threshold is 12.5) and the solids and solid surfaces also contain high alkalinity. The caustic nature of the byproduct yields concerns of long-term environmental liability and impact because leakage of this alkaline liquid from impoundments into groundwater aquifers can result in mobility of several constituents of concern, including iron, aluminum, and hydroxide ion.
Worldwide, there are about 200 million tons of bauxite residues, the vast majority of which is stored in tailings ponds. Numerous methods have been attempted to mitigate the potential environmental impacts of the residue, including washing with seawater (Ward and Summers, 1993; Menzies et al., 2004), land application as a soil amendment (Lombi et al., 2002; Hughes, 2003), beneficial use as an admixture in cementitious materials (Singh et al., 1997; Zhihua et al., 2003), treatment of acidic mine drainage (Doye and Duchesne, 2003), and sewage effluent treatment (Lopéz et al., 1998). However, large-volume, economically-viable applications for dealing with bauxite residue have, as yet, not been identified.
The primary source of alkalinity in bauxite residue is NaOH remaining from the Bayer extraction of alumina. Other, more resilient, sources of alkalinity also impact the effectiveness of bauxite residue carbonation. Nikraz (2007) reported in Journal of Materials in Civil Engineering 19, Issue 1, pp. 2-9 that the effectiveness of carbonization treatment of the byproduct slurry is a function of the level of solid alkalinity present in the slurry as tri-calcium aluminate (TCA-6). However, TCA-6 is slow to react to be of any value as a sequestration reagent.
In geological sequestration, alkalinity buffering can be accomplished through mineral reaction within the host rocks, as noted by one of the inventors in Allen et al., Fuel Process. Tech. 86, pp 1569-1580 (2005). However, due to sluggish reaction rates, these mineral reactions are likely to take a long time.
A need exists in the art for above-ground carbon sequestration methods of CO2 and/or SO2 by utilizing secondary production streams such as caustic bauxite tailings and acidic brine liquors. The methods should result in the treatment of the oxides as limiting reagents, with a concomitant decrease in volume and decrease in toxicity of all waste streams involved.